A primary function of the kidneys is to maintain acid-base homeostasis. This involves dual roles of the kidneys, filtered bicarbonate reabsorption and new bicarbonate generation through the process of net acid excretion. The proximal tubule reabsorbs approximately 80% of filtered bicarbonate, and the basolateral membrane protein, Na-bicarbonate cotransporter, electrogenic, isoform 1 (NBCE1), is critical in this process. The proximal tubule also has a major role in new bicarbonate generation and net acid excretion because it is the primary site of renal ammonia production. Ammonia production and excretion is quantitatively the major mechanism of net acid excretion and accounts for almost all of the response to acid-base disturbances. We recently identified that NBCE1 may have a previously unrecognized central role in the regulation of proximal tubule ammonia production. People with familial proximal renal tubular acidosis, a genetic disease almost always due to mutations in NBCE1, have abnormal basal and acidosis-stimulated ammonia excretion. We have identified that mice with NBCE1 deletion also have abnormal ammonia excretion, and that there are specific abnormalities in the expression of each of the major proteins involved in proximal tubule ammonia production and transport in these mice. The overall aim of this application is to determine the mechanism through which NBCE1 regulates proximal tubule ammonia metabolism and to determine its role in the renal response to metabolic acidosis and hypokalemia. We will use a variety of experimental approaches, including whole animal physiology, perfused tubule studies with measurement of intracellular pH and examination of cultured proximal renal tubule cells. The net result of these studies will be to substantially advance our understanding of acid-base homeostasis by identifying and understanding a critical mechanism in the regulation of proximal tubule ammonia metabolism, and thereby in acid-base homeostasis.